Acidity of HOCN, HSCN, HNCO, HNCS: A treatment from the viewpoint of ab initio approach

The electronic structures of the molecules HOCN, HSCN, HNCO, HNCS and the anions OCN -, SCN - have been investigated ab initio at the RHF/6-31G(d) RHF/6-31G(d,p), MP2/6-31G(d)//RHF/6-31G(d) and MP2/6-31G(d,p)//RHF/6-31G(d,p) theory levels. The thermodynamic stability of the HNCO and HNCS molecules was shown to be higher than that of the HOCH and HSCN molecules, respectively. The following series of the alteration of the protolytes strength HSCN > HOCH HNCS > HNCO, HOCN > HNCO, HSCN > HNCS was substantiated. The computations taking into account the electronic correlation (MP2/6-31G(d)//RHF/6-31G(d) and MP2/6-31G(d,p)//RHF/6-31G(d,p)) reflect the general sequence of change in the pro- ton-donor properties: HSCN > HOCN > HNCS > HNCO, coinciding with the order of descending hydrophobicity of the compounds. The comparative proton-donor ability of the above acids in aqueous solutions is determined basically from the electronic structure and size of their molecules and anions OCN -, SCN -, but not on medium effects.